This project will correlate surface markers on thymus-derived lymphocytes with functional parameters, such as mitogen responsiveness, corticosteroid sensitivity, and ability to initiate graft-versus-host reactions. Methods have been developed to measure the relative amount of theta antigen per cell in different populations, and to identify which cells have been activated by mitogens or alloantigens in mixed populations. These methods will make it possible to identify the cells responding in synergistic mixtures of young and old T lymphocytes. We have observed a T-T cell interaction in responses to phytomitogens. Preliminary evidence indicates that this is an interaction between immature T cells -- possibly even stem cells -- and mature T cells of the recirculating type. It seems that th immature cells "help" the mature ones in this response. The mitogens have been shown to induce differentiation of T stem cells into more mature (theta-positive) T cells. The functional activity of these cells will be examined.